1. Field of the Invention
As is well known, electrolytes can be separated from nonelectrolytes in solution therewith using any of a number of chromatographic techniques including: ion exchange, ion exclusion, and ion retardation. Ion exchange systems, in which ions are exchanged between the solute and the resin bed, have found wide application in industry due mostly to the systems ability to handle relatively high flux rates and a plethora of ionic species. However, because ion exchange does take place, regeneration of the resin is required. Ion exchange resins are typically classified as strongly or weakly acidic or strongly or weakly basic. Strongly acidic resins usually contain sulfonic acid groups, whereas weakly acid resins usually contain carboxylic acid groups. Strongly basic resins usually have quaternary ammonium groups while weakly basic resins usually contain polyamine groups.
An ion exchange resin with interchangeable Na.sup.+ ions is said to be in its sodium form. Introducing an electrolyte such as an aqueous solution of H.sub.2 SO.sub.4 to the system results in an exchange of the Na.sup.+ with H.sup.+ ions and convert the resin to its hydrogen form resulting in an elution of Na.sup.+ from the column. The subsequent elution of H.sup.+ ions from the column, commonly known as "breakthrough," indicates that the column resin has been mostly or fully spent. As may be appreciated, prior to the addition of more acid, such spent resin must be regenerated to its sodium form.
Ion exclusion systems, sometimes referred to as electrolyte exclusion systems, employ the same resins used in ion exchange systems, discussed supra, but differ in that the ionic functionality of the resin is the same as that of the electrolyte and, therefore, there is no exchange of ions. As will be appreciated, resins used in the instant invention are typically sulfonated polystyrenes with some degree of divinylbenzene (DVB) cross-linking which imparts physical stability to the resin polymer. The sulfonic acid functionality of the resin particles causes swelling in aqueous media. The resulting microporous resin particles can sorb water and nonionic solutes. The degree of molecular cross-linking with DVB influences the extent of sorption and prevents total dissolution of the porous resin. Because of ion repulsion and a high fixed acid chemical potential inside the resin microstructure, an electrolytic species, such as sulfuric acid in an acid/sugar mixture, for example, is effectively prevented from entering the porous resin. However, the nonionic sugar molecules are free to diffuse into the resin structure. Thus, electrolytes will pass through a packed resin bed faster than nonelectrolytes which are held up or delayed within the resin's microporous structure. In applying the instant invention to effect an acid separation similar to the separation used in the acid exchange system, supra, the resin used would be in its hydrogen form as opposed to the sodium form and, therefore, no ion exchange would occur in the system.
At the present time, ion exclusion technology is used for separation of ionic from nonionic or strongly ionic from weakly ionic solutes in polar media in certain analytical procedures, glycerin purification, and mixed acids separations applications (R. W. Wheaton and W. C. Bauman, Annals New York Academy of Sciences, 1953, Vol. 53, pp. 159-176). It differs from conventional ion exchange in that there is no net ion exchange between solute and resin. This eliminates the need for resin regeneration. Ion exclusion technology appears to have utility in separating ionic from nonionic species in aqueous solutions (D. W. Simpson and R. M. Wheaton, Chemical Engineering Progress, 1954, Vol. 50, No. 1, pp. 45-49). Basically, the ionic species are excluded from the fluid within the resin because of ionic repulsion within the resin particle micropore structure. This phenomena is explained by the Donnan exclusion principle, supra. Contrastingly, the nonionic species have no ionic repulsion with the resin and, therefore, penetrate the fluid within the porous resin to a greater degree. Thus, when a mixture of these two species is passed through a column of ion-exchange resin, the ionic component elutes first because it is excluded from the resin structure micropore volume. The nonionic species elutes after the ionic component because it has penetrated the resin micropore volume.
The physical and chemical characteristics of the resin are of vital importance to the design of an ion exclusion process. The total resin packed column volume can be thought of as to consist of three primary zones: 1) the macropore, also called void or interstitial volume, V.sub.o, which is the liquid volume between the resin particles, 2) the micropore volume, also known as occluded volume, V.sub.p, which is the liquid volume held within the resin particles, and 3) the solid resin network volume, V.sub.r, which is the actual structure of the resin (S. R. Nanguneri and R. D. Hester, Separation Sci. & Tech., 1990, Vol. 25, pp. 1829-1842). Due to the inherent ionic nature of the resin, an unequal distribution of ionic solute species exists between the micropore fluid (inside the resin) and macropore fluid (outside the resin) fluid phases. Thus, different resins with different pore volumes, ionic functionalities, and ionic charge density exhibit different separation characteristics with different solutes.
Ionic species which do not penetrate or slightly penetrate into the resin micropore volume have distribution coefficients close to zero. Nonionic species which can penetrate the resin micropore volume have distribution coefficients greater than zero but less than one. If a chemical affinity exists between a species and the resin, then the distribution coefficient can exceed one.
2. Description of the Prior Art
Ion exclusion, though widely used in analytical and pharmaceutical applications for many years, was not considered until recently for use in other than such applications due to the relatively low flux rates, small feed volumes, and weak electrolyte concentrations required to minimize dispersion and, thereby, provide for good species separation of the feedstock solution. Also, exacerbating the deleterious effects of dispersion caused by high flux rates, large feed volumes, and strong electrolyte concentrations was the dispersion caused by the presence of a so-called dead volume above the resin bed. Such dead volume resulted from shrinkage of the resin bed caused by the presence of a strong electrolyte such as sulfuric acid. Although identified as the primary factor contributing to dispersion, no successful means was devised until the discovery comprising the copending application of Hester et al., Ser. No. 08/128,174, filed, Sep. 29, 1993, to deal with this phenomenon of dead volume caused by resin shrinkage. For purposes of teaching, disclosing, and claiming the instant invention, the teachings, disclosure, and claims of said reference, supra, to wit, Hester et al., are herewith and hereby incorporated herein by reference thereto.
The possibility of using strongly acidic cation exchange resins for the separation and recycle of acid from synthetic solutions of glucose and sulfuric acid has been investigated (R. P. Neuman et al., Reactive Polymers, 1987, Vol. 5, pp. 55-61). The work conducted at that time using Rohm and Hass Amberlite IR-118 resin in the hydrogen form and using small columns demonstrated the potential for this type of process chromatography. Note: Any reference made herein to materials and/or apparatus which are identified by means of trademarks, trade names, etc., are included solely for the convenience of the reader and are not intended as, or to be construed, an endorsement of said materials and/or apparatus. Although no actual hydrolyzates were used in the work reported by Neuman et al., the synthetic solution containing 7.7 percent H.sub.2 SO.sub.4 and 1.0 percent glucose showed separation of glucose from sulfuric acid at sample loading of 10 percent of the interstitial (column void) volume and at temperatures of 55.degree. C. and 68.degree. C. However, as noted by the authors, this work confirmed the potential for significant dispersion when operating even small ion exclusion systems.
The techniques revealed in the invention described and taught in the copending application of Hester et al., supra, readily lend themselves to batch applications, whereas the techniques revealed in the teachings of the instant invention are directed primarily to practice in semi-continuous or continuous applications, such as simulated moving bed (SMB) technology. SMB systems such as the Shanks merry-go-round have been applied in adsorption and ion exchange systems for many years. The Shanks system for leaching soda ash was introduced in England in 1841. The use of SMB or merry-go-round systems is quite common in the pharmaceutical industry as described in: (J. W. Chen et al., Ind. Eng. Chem. Process Des. Devel., 1972, Vol. 11, p. 430); for activated carbon adsorption in the chemical industry (H. J. Fornwalt and R. A. Hutchins, Chem. Eng., May 9, 1966, Vol. 73, No. 10, pp. 155-160) and (M. J. Humenick, Jr., "Water and Wastewater Treatment," Calculations for Chemical and Physical Processes, Marcel Dekker, New York, chap. 6, 1977); for ion exchange in uranium purification (M. Streat, J. Sep. Process Technol., 1980, Vol. 1, No. 3, p. 10); and for waste water treatment with activated carbon (R. L. Culp et al., Handbook of Advanced Wastewater Treatment, 2nd ed., Van Nostrand-Reinhold, New York, chap. 6, 1978), and (C. T. Lawson and J. A. Fisher, AIChE Symp. Ser., 1974, Vol. 70, No. 136, p. 577), and (J. D. Parkhurst et al., Water Pollut. Control Fed. J., 1967, Vol. 39, No. 10, Part 2, pp. R70-R80). The primary advantages of SMB or similar systems in ion exclusion are the lower requirements (i.e., reductions of greater than 50 percent) for amounts of resin, water, and energy.
In work conducted at a time prior to the making of the instant invention, new methods and means were quite unexpectedly discovered to overcome or compensate for the deleterious effects of dispersion, which new methods and means much more efficiently utilized ion exclusion technology to separate ionic components from nonionic components in solution than were taught in the art prior thereto. This most recent advance in the prior art is discussed and claimed in greater detail in Hester et al., supra. Hester et al. teach utilizing a standard column or, more preferably, a combination of several columns operated in series hydraulic order which columns are adapted with a specially designed fixed or, more preferably, a specially designed movable or floating head distribution plate to effectively eliminate dispersion normally resulting from dead volume effected by the shrinkage of the ion exclusion resin when exposed to strong electrolytes such as sulfuric acid. In a principal embodiment envisioned by Hester et al. one or more columns of ion exchange resin, converted to its hydrogen form, hence ion exclusion resin, are subjected to repeated introduction at or into the uppermost regions thereof of hydrolyzate, nominally aqueous mixtures of sugar and sulfuric acid, followed by water washing, wherein separation therebetween is effected by the elution of the ionic component ahead of and completely apart from the nonionic component. In a convenient depiction thereof each column, in the case of a series of columns being utilized, is subjected to introduction of both the hydrolyzate and the wash water through introduction of same into or near the top of the floating head assembly and onto the upper surface of the resin bed followed by elution of the separated components at or near the bottom of the column. As taught in this most recent prior art, the force of gravity may be utilized to effect juxtaposition of the bottom surface of the floating head with the top or upper region of the resin bed so that as said resin bed shrinks, the so-called floating head is urged to stay in continual contact or close proximity therewith. For convenience, in said invention the preferred embodiment described introduction of the hydrolyzate and the wash water through the floating head and onto the top of the resin bed. This arrangement proved to be generally desirable in that it provided a readily useful method and means for effecting distribution in a more or less uniform manner of the input liquid onto the top of the resin bed. Also referenced in the copending application of Hester et al. is the teaching of utilizing a plurality of such columns in an arrangement known in the art as a simulated moving bed. For those who are not comfortably familiar with the art relating to large-scale or commercial-scale adsorption and chromatography applications, attention is directed to Large-Scale Adsorption and Chromatography, Volume II, Phillip C. Wankat, Ph.D., CRC Press, Inc. Boca Raton, Fla. 1986.